Fix connection migration cause when alternate network comes up after path is degrading.

Fix connection migration cause when alternate network comes up after path is degrading so that data is collected in Net.QuicSession.ConnectionMigration.OnPathDegrading instead of Net.QuicSession.ConnectionMigration.OnNetworkConnected.

Bug: 804048
Change-Id: Ic200bb845388ac10d414c3377d1f65d2c4fc8836
Reviewed-on: https://chromium-review.googlesource.com/1068034Reviewed-by: Ryan Hamilton <rch@chromium.org>
Commit-Queue: Zhongyi Shi <zhongyi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562053}

net/quic/chromium/quic_chromium_client_session.cc

@@ -1816,7 +1816,12 @@ void QuicChromiumClientSession::OnNetworkConnected(
   if (!migration_pending_ && !connection()->IsPathDegrading())
     return;
 
-  current_connection_migration_cause_ = ON_NETWORK_CONNECTED;
+  if (connection()->IsPathDegrading()) {
+    current_connection_migration_cause_ = ON_PATH_DEGRADING;
+  } else {
+    current_connection_migration_cause_ = ON_NETWORK_CONNECTED;
+  }
+
   if (migrate_session_on_network_change_v2_) {
     LogHandshakeStatusOnConnectionMigrationSignal();